Radio frequency transmissions take many forms, and some are particularly useful in wireless networks. In spread spectrum transmissions a signal is spread over a wider band than the original bandwidth (usually narrow band) of the signal. One example of a spread spectrum transmission method is the frequency hopping spread spectrum (FHSS) method in which a narrow band signal to be transmitted is combined with a carrier which makes a sequence of pseudo-random changes or “hops” in frequency. The sequence is known to the transmitter and receiver, so the original signal can be retrieved at the receiver. Another example is the direct sequence spread spectrum (DSSS) method in which an original (narrow band) signal is multiplied by a pseudo-random “noise” sequence signal. Again, the sequence is known to transmitter and receiver.
Problems may arise when these types of transmission are attempted in close proximity, as interference can occur. Interference is particularly likely to occur if both transmissions are using the 2.4 GHz band, which may be desirable for many applications. One previously considered scheme for reducing interference is for the system using FHSS to rule out the use of any frequencies or channels coinciding with or near the frequencies or channels of the system using DSSS so that none of the hops will be to a channel or frequency overlapping or nearby one in use. However, this previous scheme is typically used only in low power FHSS systems (at, for example, 0 dbi—zero antenna gain in decibels-isotropic) where the FHSS signal is unlikely to affect the DSSS signal and cannot be used for high power signals. In addition, the original signal to be transmitted by these FHSS systems is usually very narrow band, as the bit rate is low (for example, only around 1 Mb required). Higher bit rates cannot be accommodated. Some previous FHSS systems require the suppression of the DSSS signal when the FHSS system is activated, for example where the signals are generated by the same apparatus. These previous schemes require that large parts of the bandwidth for FHSS are not used, as whole channels or frequency ranges near those of the DSSS signal are ignored.